sequence.ts (Maybe only atm)

sequnce.maybe.js

import { Maybe } from "monet";

/**
 * Turns Maybe<T>[] into Maybe<T[]>
 * This is basically Ramda's sequence; http://ramdajs.com/docs/#sequence
 * But their types don't let us use it *angryface*
 * @param xs List of maybes
 */
export function sequence<T>(xs: Maybe<T>[]): Maybe<T[]> {
    return xs.reduce<Maybe<T[]>>((acc, x) => {
        //turn the Maybe<T> into a Maybe<(ts: T[]) => T[]>
        //which, via a closure, adds T to the T[]
        const append = x.map(x1 => (xs1: T[]) => [...xs1, x1]);

        //apply (ap = apply) the contents our of Maybe<T[]> to the above function
        return acc.ap(append);
    }, Maybe.of([]));
}